A former coral island which has been eroded and subsided below the surface of the ocean is known as?

The correct term for a former coral island that has been eroded and submerged below the surface of the ocean is a seamount. A seamount is an underwater mountain formed by volcanic activity. In the context of coral islands, they can form over time as coral builds up on a volcanic island, but when that volcanic island erodes or subsides, the coral structure may remain as a seamount beneath the ocean surface.

Atolls, on the other hand, are typically formed from reef structures surrounding a lagoon and do not imply that the original island is completely submerged. A submerged waterfall refers to a phenomenon where water falls over the edge of an underwater geography, which does not apply to the erosion of islands. Ocean eruption would involve volcanic activity, which doesn’t accurately describe the process of erosion and subsidence of coral islands. Therefore, the term seamount is the most fitting for this geological process.